Exemplary embodiments relate to a nonvolatile memory device and a method of programming the same and, more particularly, to a nonvolatile memory device and a method of programming the same, which are capable of improving a distribution of the threshold voltages of memory cells.
The program operation of a nonvolatile memory device is performed to raise the threshold voltage of a selected memory cell by supplying a program voltage to a word line coupled to the selected memory cell.
In single level cells (SLCs) having a single level, a threshold voltage period is divided into two periods—an erase state and a program state. In such a threshold voltage period, reliability of the device is not greatly affected because the interval (or margin) between a distribution of the threshold voltages of cells being in the erase state and a distribution of the threshold voltages of cells being in the program state is not great.
Multi-level cells (MLCs) having multiple levels and having a number of distributions of the threshold voltages have recently been developed. Therefore, it becomes an important issue to secure the margin between different distributions of the threshold voltages of the cells.
FIG. 1 is a timing diagram illustrating a known program operation. This figure shows voltages supplied to a selected word line Sel.WL coupled to a selected memory cell. A program operation using, for example, an incremental step pulse program (ISPP) method is described below.
In the program operation, a distribution of the threshold voltage of the selected memory cell may be widened because periods T1, T3, and T5 in which program voltages Vpass are supplied, and periods T2 and T4 in which verification operations are performed, are repeatedly performed. For example, during the period T1, the pass voltage Vpass is first supplied to the selected word line Sel.WL, and then a program start voltage V1 is supplied. Here, the period T1 is continued during a time D1. Next, during the period T2, a program verification operation is performed by supplying a verification voltage Vr to the selected word line Sel.WL. If, as a result of the program verification operation, the program operation has not been completed, a program voltage V2 having a higher level than the voltage V1 supplied in the period T1 is supplied to the selected word line Sel.WL during the period T3. The period T3 is continued during a time D2. In this manner, the program voltage having a gradually rising level is supplied to the selected word line Sel.WL until the program operation is completed (within a critical number of times). The program verification operation is performed between the periods in which the program voltages are applied. Here, the times D1, D2, and D3 during which the program voltages are supplied have the same duration (i.e., D1=D2=D3).
FIG. 2 is a graph illustrating known distributions of the threshold voltages of memory cells.
It is preferred that a number of memory cells included in the same memory chip have the same operation (program or erase) speed. However, the operation speeds of actual memory cells are slightly different. For example, in the case in which a specific memory cell of the memory cells has a faster operation speed than the remaining memory cells, the threshold voltage of the specific memory cell can be raised to a desired voltage level although a program voltage having a low voltage level is supplied to the specific memory cell. Here, since the threshold voltages of the remaining memory cells have to be raised, the program voltage is supplied to all the memory cells several times by gradually raising the level of the program voltage supplied to the memory cells. This method is called an ISPP method. As described above, the program voltage continues to be supplied to the specific memory cell having a faster operation speed than the remaining memory cells. Accordingly, a distribution of the threshold voltages of the memory cells can be increased (indicated by ‘W’ in FIG. 2). In particular, in the case of MLCs, reliability of the device can be deteriorated according to a reduction in the interval (or margin) between distributions of the threshold voltages of the cells.